postsettlement patterns of habitat use by sciaenid … patterns of habitat use by sciaenid fishes in...

Estuaries Vol. 21, No. 2, p. 318-327 June 1998

Postsettlement Patterns of Habitat Use by Sciaenid

Fishes in Subtropical Seagrass Meadows

JAY R. ROOKER~,~ SCOTT A. HOLT MANUEL A. SOT0 G. TOAN HOLT ~-J--- Matine Science Institute The University of Texas at Austin 750 Channelview Drive Port Aransas, Texas 78373

ABSTRACT: Spatial and temporal patterns of distribution and abundance were examined for postsettlement sciaenids collected from seagrass meadows in the Aransas Estuary, Texas. Overall, 5443 sciaenid larvae and early juveniles were identified from biweekly epibenthic sled collections taken from August 1994 to August 1995. Eight species were present in seagrass meadows, witb five accounting for over 99.9% of sciaenids collected: silver perch (B&die& chrysoura), spotted seatrout (Cynoscion nebulosus), spot (Lekxtomus xonth~r~~), Atlantic croaker (Micropogonias undulutus), and red drum (Sckenops ocellati). Settlement to seagrass meadows was partitioned temporally with little overlap among the five species. Postsettlers from inshore spawners (B. chrysoura, C. nebulosus, S. ocellatw) inhabited seagrass meadows during the spring aud s- er, while individuals from offshore spawners (L. xanthurus, M. undulatus) were present in the late fall and winter. Densities of B. chrysouru, C. nebulosus, S. ocellatus were highest for small individuals (4-8 mm SL) and these taxa remained in seagrass sites through the early juvenile stage. Conversely, L. xanthurus and M. undulatus maintained longer pelagic periods and generally entered seagrass meadows at larger sizes (lo-14 mm SL). Moreover, these taxa were only temporary residents of selected seagrass meadows, apparently migrating to alternative habitats shortly after arrival. During peak settlement, mean and maximum densities among species ranged from 0.1 m-r to 0.8 m-* and 0.7 m-r to 23.8 rne2, respectively. Density and mean size of postsettlement sciaenids differed significantly between seagrass species (Halodule wright& Thahzssia testudinum) and among sites within the estuary.

Introduction Seagrass meadows provide critical nursery habi-

tat for a diversity of fishes during their early life stages (Olney and Boehlert 1988; Bell and Pollard 1989). Increased abundance and diversity of fishes associated with seagrass meadows have been related to the physical complexity of the seagrass canopy. Increased complexity provides such benefits as refuge from predators (Heck and Orth 1980; Stoner 1982; Ryer 1988; Savino and Stein 1989) and increased food supply (Orth 1992). In addition, the physical nature of the seagrass canopy may affect the hydrodynamics of larval recruitment (Eckman 1983) and influence the chance of settlement by pelagic larvae (Bell et al. 1987).

Larvae of several estuarine fishes settle into seagrass meadows following a dispersive pelagic phase (Bell and Pollard 1989; Day et al. 1989; Leis 1991). Since postsettlement mortality is typically high and variable, recruitment success or failure can be

1 Corresponding author. * Present address: University of Maryland, Center for Environ-

mental Science, Chesapeake Biological Laboratory, P.O. Box 38, Solomons, Maryland 20688-0038.

0 1998 Estuarine Research Federation 318

linked to differential survival during this period (Jones 1991; Carr and Hixon 1995; Rooker 1997). As a result, studies focusing on the postsettlement phase are essential for understanding the dynamics of early life. However, detailed ecological assess- ments of habitat requirements during the postsettlement phase are incomplete for many estuarine species. This information is required for effective conservation and management of the estuarine fish-habitat complex and will provide the foundation for future studies addressing the causes and consequences of early life mortality.

Species from the family Sciaenidae (drums) are vital to the recreational fishery in the Gulf of Mex- ico, and several sciaenids exploit seagrass meadows during early life (e.g., Stoner 1983; Tolan et al. 1997). From limited data, it appears that certain sciaenids immigrate to seagrass meadows following a pelagic interval of approximately 15-30 d and remain in these habitats for weeks to months (Holt et al. 1983; Peters and McMichaell987; McMichael and Peters 1989; Rooker and Holt 1997). To date, few studies have examined postsettlement patterns of habitat use in seagrass meadows and important life history data (e.g., duration and timing of set-

Postsettlement Sciaenids in Seagrass Meadows 319

tlement, size-at-settlement, and spatial distribution) are lacking for many species.

The purpose of the present study was to examine sciaenid utilization of seagrass meadows during the postsettlement phase. Specific objectives of this research were to describe quantitatively the sciaenids inhabiting seagrass meadows in the Aransas Estu- ary, Texas, determine the temporal patterns of settlement, including the timing and duration of events, and determine the consistency of habitat use over different spatial scales.

Methods %XJDlY hEA

The study was conducted in the Aransas Estuary, _._ 0c ” 1..--9 L_..l-..:lL __.__ f~_ _. _ PA_ ~_ ~~ _ an oo.* Km', uar-Dullr eslcudryconasting 01 two ma-

jor bays (Aransas Bay, Redfish Bay) oriented par- allel to the coast. The estuary is located in the vi- cinity of the Aransas Pass tidal inlet (Fig. l), which is approximately 400 m wide and connects Gulf of Mexico waters to inshore habitats. Seagrass meadows within the estuary are dominated by shoal ~T~PEQ / Unln&110 w&rrhtl’i\ 2nd tllrtlP CTT>CC / Thnlnrrin w-“” \‘LW”“W~yY w,“~‘““y”, %+*-La GUI LA.. b’““” \ 11 ,Y..“vIyUYYW

testudinum). Both species occur primarily in shallow water (ca. <l m), low-energy areas and are considered the major primary producers in these areas (Zieman and Wetzel 1980). Water temperature, salinity, and turbidity are variable within the estuary; however, our study sites were in close prox- imitv to the tidal inlet (<lo km) and maintained con&tions relatively similar to nearshore waters in the Gulf of Mexico.

POSTSETTLEMENT SURVEY Densities were estimated by enumerating sciaen-

ids in biweekly samples taken from August 1994 to August 1995 (postsettlers were defined as individ- uais 140 mm standard iength collected in seagrass meadows). Collections were taken at six sites within the estuary (Fig. 1): Aransas Bay 1, 2, and 3 (ABl, AB2, Al33); Redfish Bay 1 and 2 (RBl, RB2); and Corpus Christi Bay (CBl). At each of the six locations, H. wrightii meadows were sampled. At one site within each bay (ABl, RBl, CBl), co-occurring, monotypic stands (>90% single species) of H. wrightii and x testwdinum were present (des- ignated paired sites) and sampled separately. Sea- grass meadows at paired sites were adjacent to one another and less than 50 m apart.

Postsettlement sciaenids were collected from seagrass meadows with an epibenthic sled. The sled opening measured 0.75 m (length) X 0.50 m (height) and was equipped with a 505~pm conical plankton net. The sled was towed by hand at a con- stant speed (-0.7 m s-l) through seagrass meadows. Triplicate sled tows were made at each sampling site on each trip. Sled tows were 20 m in

length and the area of seagrass sampled per tow was 15.0 m2. Individuals were preserved in 95% ethanol immediately after capture. Standard lengths (SL) were measured in the laboratory to the nearest 1.0 mm.

Throughout the postsettlement survey, environmental conditions were recorded in the Aransas Pass at the University of Texas, Marine Science In- stitute Pier Laboratory. In addition, depth, salinity, and water temperature were measured from each site on all sampling trips. Estimates of seagrass complexity were also made from samples collected with a 7.5-cm diameter coring device. Shoot density, blade height, and number of blades per shoot were measured from three replicate cores taken K,-,.- ,,,.1, n:,, ,e ?z n -_ :,c-,.,“l” l?,c:-..C,.” ,c II”lll C;aL.ll JILr; aL J1 ill” IIILC1 “al>. IjJUllldLCJ “I blade height and blades per shoot for each core were mean values based on 10 blades and shoots, respectively.

STATISTICAL ANALYSIS Three-way factorial analysis of variance (ANO-

VA) was applied to postsettlement density data (main effects: habitat, site, date). This analysis was performed separately on each taxon and based only on samples from paired sites (ABl, RBl, CBl) . Two-way factorial ANOVA (main effects: site, date) was used to examine variation in density among the six H. wrightii sites (ABl, AB2, AB3, RBl, RB2, CBl). Prior to ANOVA testing, density data were log, (x + 1) transformed to minimize heteroscedasticity. Since seasonal patterns of recruitment were pronounced, many samples con- tained zero values. As a result, the assumption of multivariate normality (normality of error terms) was not met in some cases. ANOVA is robust to most types and magnitudes of departure from normaiity and, therefore, non-normaiity is uniitiy to compromise results (Underwood 1981). Tukey’s HSD test was used to find a posteriori differences (CY = 0.05) among sample means. Mean sizes of sciaenids in H. wrightii and ?: testudinum were com- pared using a one-way ANOVA. Due to heteroscedasticity and unequal sample sizes, means were weighted by the reciprocal of the sample variances of the group means (JMP 1989).

Results ENVIRONMENTAL CONDITIONS AND

SITE CHARACTERISTICS Temporal variation in water temperature and sa-

linity in the Aransas Pass were pronounced (Fig. 2) ; temperature and salinity ranged from 12.2”C to 31.1”C and 22.5%0 to 35.7%0, respectively. Mea- surements were also recorded each trip at the different sampling sites within the Aransas Estuary; however, it is difficult to compare temperature and

320 J. FL Rooker et al.

n

^^____^ CiJHPiJS 0

CHRIST/ BA Y

,27’54’

.27”50

97'06'

Fig. 1. Map of Aransas Estuary showing location of sampling sites in Aransas Bay (ABl, AB2, AB3), Redfish Bay (RBI, RB2), and Corpus Christi Bay (CB-1).

salinity profiles among sites due to the time-of-day effect. In general, temperature changes in the Aransas Estuary closely matched recordings at the Aransas Pass, but larger fluctuations were apparent in the shallower waters of the estuary. Salinity mea- surements at the tidal pass were representative of conditions at study sites within the estuary.

Monotypic stands of H. wrightii and ?: testudinum differed in physical structure and complexity. On

average, lY testudinum was present at depths lo-20 cm > H. wrightii (0.58 m and 0.44 m, respectively). Shoot densities in H. tightii were approximately 5-10 times higher than ‘T testudinum during the summer and fall, while blade heights were generally lower. Shoot densities and blade heights varied among sites and seasonal patterns were observed (Table 1). The most salient feature was the marked decline in the shoot density and blade height of


- Temperature - Salinity

A S 0 N D J F M A M J J A 2o

Date

Fig. 2. Weekly water temperature and salinity records for the Aransas Pass from August 1994 to August 1995.

H. wrightii during the winter (Table 1). Number of blades per shoot was similar between H wr;,ghtii (2.6 + 0.7 SD) and ?: testudinum (2.4 2 0.7 SD).

ICHTHYOFAUNAL COMPOSITION OF SEAGRASS MEADOWS

The taxonomic composition of ichthyofauna from seagrass meadows in the Aransas Estuary was diverse, with 54 species from 26 families (Table 2). Individuals from four families composed over 85% of the total catch: Sparidae (27.4%), Gobiidae (22.8%)) Sciaenidae (18.6%)) and Syngnathidae (17.6%). Numerical dominants were pinfish (Lu- godon rhomboides) (27.1%) , darter goby (Co&one&s bokosoma) (16.6%)) gulf pipefish (Syngrzathus scov- eZZi) (15.2%)) and a suite of sciaenids.

Eight species from the family Sciaenidae were present in seagrass samples (Table 3). Of these, five species were abundant and accounted for over 99.9% of sciaenids collected: silver perch (BairdieZ- la chrysowu) , spotted seatrout (Cynoseion nebulosus) , spot (Leiostomus xunthurus) , Atlantic croaker (Mi- cqbogonias undulatus) , and red drum (Sciaenops ocellatus) . Sciaenids collected in seagrass meadows

with the epibenthic sled were almost exclusively postsettlers; 98.4% of sciaenids collected were 540 mm. Further quantitative description will be limited to postsettlers of these five species.

TEMPORAL VARIATION IN SETTLEMENT

Settlement events (based on densities of postsettlement sciaenids) were species-specific and partitioned temporally (Fig. 3). Newly settled C. nebu- Zosus were present in seagrass meadows from late spring through summer (April to October); densities were highest in August. Sciaeno@ ocellatus entered seagrass meadows in late August and highest densities were reached from late September through October. Shortly thereafter, M. undulatus dominated seagrass meadows and settlement was bimodal, with two primary periods: November and February. Similarly, the settlement of L. xunthurus was characterized by two peaks (December, March), each following M. undulatus peaks by 2-4 wk. The number of new settlers was highest for M. undulatus and L. xanthurus during the first (No- vember) and second (March) settlement pulse, respectively. From April to June, B. chrysoura was the

TABLE 1. Mean shoot density (shoots m-2) and blade height (cm) of seagrass meadows in the Aransas Estuary. Estimates of seagrass complexity are given by site, habitat, and date. Values in parentheses represent standard deviations.

Site

ABl. Al31 AB2 AB3 CBl CBl RBl RBl RB2 All sites All sites

June 27, 1994 November 11, 1994 February 27, 1995

Density Height Density Height D.ZlSi~ Height

8,773 (4,402) 19.7 (2.9) 5,094 (0) 14.5 (1.6) 943 (865) 4.0 (0.0) 377 (163) 24.6 (4.5) 849 (283) 20.6 (8.5) 849 (490) 14.2 (3.3)

7,169 (2,198) 12.9 (5.2) 4,528 (1,201) 14.7 (2.3) 472 (423) 6.3 (0.6) 8,207 (849) 15.0 (3.3) 4,151 (1,559) 13.3 (4.5) 2,358 (2,614) 8.7 (5.4)

11,131 (1,424) 12.9 (3.1) 8,301 (1,143) 14.6 (4.8) 283 (283) 5.0 (0.0) 1,320 (865) 16.5 (2.6) 991 (200) 17.4 (3.0) 1,792 (432) 13.9 (3.7) 6,320 (1,276) 13.9 (3.0) 5,188 (432) 12.3 (5.1) 566 (980) 3.3 (1.2)

943 (712) 25.2 (4.8) 1,698 (283) 20.8 (2.4) 1,792 (432) 9.8 (3.6) 7,075 (2,136) 22.0 (3.2) 5,754 (432) 18.6 (4.9) 4,905 (2,891) 7.5 (2.4)

8,113 16.1 5,503 14.7 1,588 5.8 880 22.1 1,179 19.6 1,478 12.6

Habitat

Halodule Thalussia Haloduk Halodule Halodule Thalmsia Haloduk Thalassia Halodule Halodule Thalassia

322 J. R. Rooker et al.

TABLE 2. Number and percent composition of fishes (by fam- TABLE 3. Number and percent composition of sciaenids col- ily) collected from seagrass meadows in the Aransas Estuary. lected from seagrass meadows in the Aransas Estuary.

Family Number PelX.Zllt

Sparidae 8,014 27.37 Gobiidae 6,686 22.83 Sciaenidae 5,443 18.59 Syngnathidae 5,149 17.59 Cyprinodontidae 1,046 3.57 Clupeidae 894 3.05 Haemulidae 693 2.37 Mugilidae 465 1.59 Gerriidae 390 1.33 Atherinidae 179 0.61 Engraulidae 116 0.40 Cynoglossidae 38 0.13 Bothidae 38 0.13 Lutjanidae 33 0.11 Blenniidae 32 0.11 Ophichthidae 15 0.05 Bauachoididae 12 0.04 Synodontidae 11 0.04 Other taxa 26 0.08 Total 29,280 100

Species Common Name Number Percent=

Leiostomus xanthurus spot 2,015 37.0 Bairdiellu chrysoura silver perch 1,271 23.4 Sciaenops ocellatus red drum 1,170 21.5 Micropogonias undulatus Atlantic croaker 723 13.3 Cynoscion nebubsus spotted seatrout 258 4.7 Pogonias cromis black drum 4 0.1 Cynosn’on arenarius sand seatrout 1 co.1 Menticirrhus ama’canus southern kingfish 1 co.1

a Based on total number of sciaenids collected.

50.

40.

30.

Cynoscion nebutosus

dominant sciaenid in seagrass meadows with peak densities occurring in May.

Sciaenops ocektus

SIZE-AT-SETTLEMENT

Bairdiella chrysoura, C. nebulosus, and S. ocellatus were first detected at approximately 3-4 mm and peak numbers of settlers entered seagrass meadows at 4-8 mm (Fig. 4). Individuals from these species were present at collection sites throughout much of the early juvenile stage (540 mm). In contrast, newly settled L. xanthurus and M. undulatus were considerably larger. Although individuals <lo mm were collected in seagrass meadows, densities were highest for individuals lo-14 mm. Moreover, L. xanthurus and M. undulatus remained in these habitats for short periods of time before moving to other habitats; individuals >20 mm were caught infrequently.

Micropogonias undulatus

Leiostomus xanthurus

SPATIAL DISTRIBUTION AND ABUNDANCE

Densities of postsettlers varied markedly between habitats Andy among sites at paired sites (ABl, RBl, CBl). A significant habitat effect was observed for three species: B. chrysoura (F1,*s8 = 9.48, p = O.OOl), C. nebulosus (F1,288 = 4.07, p = 0.045), and S. ocellatus (F1,288 = 18.11, p < 0.001). Densities of C. nebulosus and S. ocellatus were approximately 2-3 times higher in H. wrightii than ?: testudinum. Mean densities of S. ocellatus during the primary settlement period (defined as trips with >50% of samples with target species) were 0.34- 1.79 mm2 and 0.04.0.73 mP2 in H. wrightii and ?: testudinum, respectively (Table 4). Although mean densities of C. nebulosus were lower, trends were similar: H. wrightii (,0.04-0.21 mm2) and X testudin-

Bairdiella chrysoura

1994 1995 Date

Fig. 3. Relative abundance of sciaenids (540 mm SL) collected from seagrass meadows during biweekly sampling trips in the Aransas Estuary. Percent frequency estimates based on the number of individuals per sampling date.

Sciaenops ocellatus

12 10

6- 6- 4- 2- O-

Micmpogonias undulatus

16. 16. 14.

12. lo-

6-

6-

Leiostomus xanthurus

_lll I

16 16 14 12


Standard length (mm)

Fig. 4. Length distributions of sciaenids (540 mm SL) collected from seagrass meadows in the Aransas Estuary. Percent frequency estimates based on the number of individuals per l- mm size class.

urn (0.01-0.07 mm2). Conversely, densities of B. chrysoura were significantly higher in T. testudinum with over 70% collected from this habitat. Mean densities ranged from 0.12 m-* to 1.01 m-* in H. wrightii and 0.04 m-* to 2.61 mm2 in T testudinum. Although mean densities of L. xanthurus and M. undulatus were markedly higher in lY testudinum, significant differences between habitat types were not detected (F,,,,, = 3.27, p = 0.072 and F1,288 = 1.27, p = 0.261, respectively). Densities of L. xanthurus and M. undulatus during the primary settlement period ranged from 0.00 mm2 to 9.80 m-* and 0.00 m-* to 23.80 m-*, respectively. Significant (p


< 0.05) habitat X date interactions were observed for all five species, indicating that the magnitude of differences between habitats was not consistent over time.

Densities of all five species differed significantly among sites (Table 4). At paired sites, densities of B. chrysoura ( F2,,,, = 98.90, p < O.OOl), C. nebulosus F2,266 = 42.49, p < O.OOl), L. xanthurus (F2,288 = 35.09, p < O.OOl), M. undulatus (F,,,,, = 19.88, p < O.OOl), and S. ocellatus (F2,288 = 45.16, p < 0.001) were significantly different among the three bay sites: ABl, RBl, and CBl. Postsettlers of S. ocellatus, B. chrysoura, L. xanthurus, and C. nebulosus were significantly more abundant at ABl than RBl or CBl (Tukey’s HSD, p < 0.05). Densities of M. undulatus were significantly lower in CBl than ABl or RBl (Tukey’s HSD, p < 0.05). Again, significant first- order interactions with date (site X date) were

l?_- ___I- --__:__ _..---_Ll._- LI^_L Al-^ -_l_ pr-esenr I01 eacr1 spc’c1cs, suggcsLIIlg LIldL LIIC lCI&

tive magnitude of postsettlement densities among sites was not uniform over sampling dates. Varia- tion in settlement density among the six H. wrightii sites was also examined separately and, similar to results described above, significant (p < 0.01) site effects were observed for all five species. In gen- eml. &nsiti_e!: ycye higheg in_ Aransas l&q partic- _- --, _ - -__-__- ularly sites ABl and AB2.

The mean size of postsettlement sciaenids found in H. wrightii and T. testudinum also varied. Mean lengths of two species, M. undulatus and S. ocellatus, differed significantly (p < 0.001) between habitats (Table 5). Postsettlement M. undulatus were markedly smaller in H. wrightii (12.6 mm) than T. testudinum (14.1 mm). A similar trend was observed for S. ocellatus; however, the size difference between habitats was larger. Mean lengths of S. ocellatus were 9.5 mm in H. wrightii and 16.1 mm in T testudinum. No habitat-specific differences in size were observed for B. chrysoura, C. nebulosus, or L. xanthurus (p > 0.05).

Discussion Ichthyofauna utilizing seagrass meadows in the

Aransas Estuary were typical of assemblages reported in the northern Gulf of Mexico (Stoner 1983) and southeastern Florida (Brown-Peterson et al. 1993). Of the dominant taxa, syngnathids and gobiids can be considered permanent mem- bers of the assemblage since these taxa utilize seagrass meadows over protracted periods, often present from larval to adult stages. In contrast, sciaenids and other taxa (e.g., sparids, haemulids) were only seasonal residents and annual fluctuations in densities may result from recruitment, migration, and natural mortality. These taxa appear to inhabit shallow seagrass meadows for short periods of time (weeks to months), primarily as larvae and early

Site

ABl ABl AB2 AB3 CB 1 CB 1 RB1 RB1 RB2 All sites % Frequency na

324 J. Ft. Rooker et al.

TABLE 4. Mean and maximum densities (individuals mm*) of sciaenids (540 mm SL) from different sampling sites in the Aransas Estuary during the primary recruitment episode (defined as trips with >50% of samples with target species). Percent frequency of occurrence estimates (based on presence or absence of individuals in sled tows) are shown. Values in parentheses are standard errors.

Bairdi& chrpmra Cynos&l nebuimus Leiostomus xanthuw Mzcropopnias undulattu saaaops ocelbtw

Seagrass MeaIl MZIX MGUI MU Mean MtlX MeaIl MZtX MeaIl Ma

Halodule 1.01 (0.24) 3.46 0.21 (0.05) 0.47 1.31 (0.34) 5.33 0.38 (0.22) 2.07 1.79 (0.20) 2.53 Thalessia 2.61 (0.55) 7.67 0.07 (0.03) 0.27 4.00 (1.63) 23.80 2.62 (1.27) 9.80 0.73 (0.20) 2.07 Haloduk 1.11 (0.36) 4.53 0.31 (0.08) 0.73 0.23 (0.09) 1.47 0.26 (0.08) 0.80 1.66 (0.23) 2.87 Halodule 0.01 (0.01) 0.20 0.11 (0.04) 0.40 0.31 (0.12) 1.80 0.07 (0.04) 0.33 0.90 (0.35) 2.87 Haladuk 0.12 (0.02) 0.33 0.07 (0.03) 0.27 0.22 (0.09) 1.33 0.03 (0.01) 0.07 0.34 (0.07) 0.67 Thalassia 0.04 (0.02) 0.20 0.01 (0.01) 0.01 0.29 (0.15) 2.40 0.00 (0.00) 0.00 0.04 (0.04) 0.33 Haladub 0.06 (0.02) 0.33 0.04 (0.02) 0.20 0.20 (0.08) 1.33 0.33 (0.10) 0.80 1.11 (0.64) 4.73 Thakzssia 0.11 (0.03) 0.33 0.04 (0.02) 0.13 0.06 (0.02) 0.27 0.31 (0.15) 1.47 0.33 (0.09) 0.93 Halodule 0.36 (0.18) 2.46 0.16 (0.04) 0.33 0.39 (0.14) 2.20 0.25 (0.09) 0.80 0.71 (0.12) 1.20

0.62 (0.11) 0.11 (0.04) 0.78 (0.21) 0.47 (0.16) 0.84 (0.09) 64.3 59.3 67.3 66.7 82.7

135 81 135 81 81

a Represents number of sled tows (27 per trip).

juveniles. In this study, sciaenids >40 mm were caught in limited numbers, suggesting that these individuals may migrate to different habitats. How- ever, Rooker (1997) showed that larger sciaenids (>40 mm) are more capable of avoiding the epibenthic sled and thus low collection numbers may be attributed to both sampling bias and migration activity.

Temporal variation in settlement of sciaenids to seagrass meadows was pronounced and habitat use was staggered over time. Each of the five sciaenids inhabiting seagrass meadows showed distinct settlement periods with little temporal overlap among species. Similarly, Deegan and Thompson (1985) found that in Louisiana estuaries peak occurrences of sciaenids were segregated temporally with limited overlap. Moreover, Chao and Musick (1977) suggested that the coexistence of young sciaenids in a Virginia estuary was made possible in part be- cause recruits entered nursery habitats at different times of the year. If resources within seagrass mead-

ows are in limited supply (e.g., food, space), it is possible that newly settled sciaenids are exposed to reduced interspecific competition through differences in the timing of settlement.

Variability in size-at-settlement was observed among sciaenids and appears related to the location of adult spawning grounds (i.e., distance from estuary). Bairdiella chrysoura and C. nebulosus spawn primarily in estuarine habitats (e.g., bays, channels), often in close proximity to nursery areas (Johnson 1978; Overstreet 1983a; Brown-Peterson et al. 1988). Settlers from these taxa arrive in peak numbers at relatively small sizes (4-8 mm). Con- versely, M. undulatus and L. xunthurus generally spawn offshore, several kilometers from tidal passes (Johnson 1978; Warlen and Chester 1985; Cow- an and Shaw 1988). Individuals were markedly larger (lo-14 mm) than settlers from inshore spawners, supporting the assumption that the source population was far removed from the tidal pass. The spawning strategy of S. ocellatus differs

TABLE 5. ANOVA comparisons of mean lengths of sciaenids (540 mm SL) between different seagrass habitats in the Aransas Estuary. Estimates based on length estimations from the three sites containing monotypic stands of both Halodule wrightii and Thakzssia testudinum (ABl, RBl, CBl).

Species Habitat Mean Length

” (mm) F Ratio” 0


cynosnon nebulosus

Lziostomus xanthurus

Micropogonias undulatus

SGiaenops ocellatus

Halodule Thalassia H&dub Thalassia Halodule Thalassia Halodule Thalassia Halodule Thakzssia

279 15.3 (0.53) 1.543 0.215”s 537 14.5 (0.38)

71 17.7 (1.39) 0.028 0.866”’ 43 18.1 (1.79)

528 15.3 (0.30) 0.304 0.581”’ 525 15.1 (0.30) 106 12.6 (0.23) 27.971 <O.OOl 376 14.1 (0.13) 441 9.5 (0.34) 49.855 <O.OOl 163 16.1 (0.56)

a ANOVA based on means weighted by the reciprocal of the sample variances of the group means; ns nonsignificant comparisons (p > 0.05).


from both inshore and offshore spawners. Sciaenops ocelkztus spawning locations range from coastal (i.e., near tidal passes) to offshore waters (Johnson 1978; Overstreet 1983b; Comyns et al. 1991). The pattern of size-at-settlement observed for S. ocellatus was similar to inshore spawners with peak settlement densities occurring at 6-8 mm. Recent work using hydrophones to monitor sound pro- duction associated with courtship and spawning of S. ocellatus has shown that activity is particularly high in the confines of the Aransas Pass (S. A. Holt unpublished data). Thus, it appears that some spawning sites for S. ocellatus are in coastal waters, probably in or near the Aransas Pass.

Although conspicuous differences in size-at-settlement were reported in this study, our estimates were based on size-at-capture data. As a result, the frequency of collections may have biased size-at-settlement estimates to some degree. For example, with biweekly collections there is a limit to the amount of postsettlement growth that could have occurred in seagrass meadows. Still, estimates of size-at-settlement are going to be overestimated to some degree depending on the amount of time individuals spend in the seagrass meadows prior to capture. Nevertheless, length-frequency profiles of each sciaenid examined in this study are based on a series of cohorts, which enter seagrass beds over a number of months. Since estimates of size-at-settlement are averaged across many cohorts for each taxon, bias associated with length-frequency profiles should be minimized.

Densities of newly settled sciaenids were influ- enced by seagrass spedes. Some sciaenids were significantly more abundant in one seagrass species or the other, suggesting that settlers may discriminate between habitats,, either during or after the initial settlement event. These data support studies which have shown that patterns of distribution for larvae and early juveniles are seagrass-specific (To- lan et al. 1997; Stoner 1.983; Middleton et al. 1984), but appear to challenge the “settle-and-stay” hypothesis (Bell and Westoby 1986; Bell et al. 1987). According to this hypothesis, larval recruits do not discriminate between meadows of different complexity when they settle, and redistribution only occurs within the original meadow since individuals that re-enter the plankton or migrate across un- vegetated substrata will be more vulnerable to predators. It should b’e noted, however, that pro- ponents of the “settle-and-stay” hypothesis recog- nize that it may not hold for species capable of safely migrating to different habitats. In this study, paired sites were composed of adjoining H. wrightii and 7: testudinum meadows. As a result, postsettlement migration activity may have been more likely since individuals were not required to cross areas

of bare substrate to reach different seagrass species.

Patterns in length-frequency data support the view that postsettlement migration occurs for certain taxa. Inshore-spawned B. chrysoura and C. ne- buulosus were common in seagrass meadows from larval through early juvenile stages, suggesting that these taxa settle and stay in seagrass meadows for extended periods. In contrast, individuals larger than 20 mm from offshore-spawned M. undulutus and L. xanthurus were rarely observed and apparently migrate to different habitats shortly after arrival. Since migratory behaviors were only conspicuous for species utilizing seagrass meadows in the late fall and winter, it is possible that activities are related to changes in environmental conditions (e.g., temperature, food supply).

Postsettlement densities of sciaenids varied between seagrass habitats in close proximity (~50 m) and patterns of this nature are often attributed to either food supply and/or predation pressure. Since recent studies have shown that growth and foraging conditions for certain sciaenids in the Aransas Estuary are similar between H. wrightii and ?: testudinum (Rooker et al. 1997; Rooker and Holt 1997; Soto 1997), it seems plausible to assume that patterns of abundance were directly or indirectly related to predation. Predator efficiency is often correlated with macrophyte complexity (e.g., shoot or blade density, biomass; Coen et al. 1981; Stoner 1982; Stoner and Lewis 1985; Ryer 1988; Heck and Crowder 1991). As complexity increases, the visi- bility of prey (encounter rate) declines and predator movements are restricted (Stoner 1980; Savi- no and Stein 1989), resulting in lower predator efficiency. Although H. wrightii and X testudinum differ in complexity (i.e., shoot density) and morphology (blade width and height), Rooker et al. (1998) demonstrated that the vulnerability of postsettlement red drum to predatory fish did not differ between the seagrass species. Consequently, other factors related to predation may be responsible for observed patterns. For example, habitat complexity (e.g., physical structure of seagrass canopy) and predator fields vary over spatial and temporal scales and may influence settlement patterns and postsettlement survival.

On larger scales, significant correlations between density of settlers and habitat complexity are rare (Stoner 1983; Bell and Westoby 1986; Sogard et al. 1987) and recruitment variability is often attributed to hydrodynamic processes (Miller et al. 1984; Boehlert and Mundy 1988; Shaw et al. 1988). Recent studies have elucidated the importance of hydrodynamic processes in estuarine systems (i.e., tidal flux) and suggested that such processes are responsible for spatial variability in larval supply

326 J. R. Rooker et al

and settlement (Boehlert and Mundy 1988; Pietra- fesa and Janowitz 1988; Jenkins et al. 1996). Tidal flow rates in the three channels leading from the Aransas Pass (see Fig. 1) were measured in 1995 (R. Solis, Texas Water Development Board person- al communication), and it appears that densities of sciaenids in the Aransas Estuary were positively associated with tidal flow rates. On average, flow rates in Lydia Ann Channel (Aransas Bay) were 4- 6 times higher than Aransas Channel (Redfish Bay) and post-settlement density was 3 times higher in Aransas Bay. There tvas no correiation between density and flow rate in Corpus Christi Bay; however, our only sampling site was relatively far removed and indirectly connected to the water conduit from the Arzmsas Pass. Although variation in tidal flow rates is probably a key component in the estuarine-ocean exchange mechanism, a vari- ety of nont;dai factors, both_ biological (P cr hP- y-.6’, -- havior of fish larvae) and physical (e.g., meteoro- logical forcing meclhanisms) are also important and likely to influence large-scale patterns of settlement within the Aransas Estuary.

Ackmtowledgments We thank Kathy Binney, Patti Pickering, and Cameron Pratt

for providing assistance in the laboratory and field. Lee Fuiman, Sharon Herzka, Chris Onuf, and two anonymous reviewers pro- vided valuable comments on the manuscript. This study was funded in part by grants from the Texas Higher Education Co- ordinating Board Advanced Research Program (grant no. 003658-392), Texas A&M Sea Grant Program (grant no. NBR: RIF65), Sid W. Richardson Foundation, and fellowships to J. R. R. (E. J. Lund, Julian C. Barton). This is contribution no. 1026 of The University of Texas at Austin, Marine Science Institute.

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Accepted for publication, October 14, I997

postsettlement patterns of habitat use by sciaenid … patterns of habitat use by sciaenid fishes in...

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